Glass, especially glass that is tinted, is subjected to large stresses due to non-uniform heating caused by the absorption of solar radiation. These stresses can be so great as to cause fractures or cracks to develop in the glass, which could ultimately lead to failure.
The center of the glass (COG) may have a considerably higher temperature than, for example, the edges of the glass, which are typically covered or shadowed by a frame or other architectural structure. Of course, the more tinted the glass, the greater the solar absorption, and the larger the potential temperature differential between the COG and the glass edges or other shaded areas. This results in stress, typically along the glass edges, which if greater than about 14 to about 28 MPa, could result in cracking. As such, normal practice dictates that glass be heat-strengthened or tempered to reduce the incidence of fracture. Typically, the absorbing glass pane is heat-treated or tempered so as to withstand at least about 35 MPa, or to conform with industry standards, such as ASTM E2431 (Practice for Determining the Resistance of Single Glazed Annealed Architectural Flat Glass to Thermal Loadings). Of course, this adds to the cost of manufacturing.
Like tinted glasses, electrochromic devices (hereinafter, “EC devices”) absorb significant amounts of solar radiation, especially when in a fully darkened state. To withstand the stresses or service loads associated with these temperature differentials, it is common practice to use heat-strengthened or tempered glass as the substrate for these devices. While this is a practical solution, the cost of manufacturing devices based on these substrates is expensive. It is desirable to reduce costs and increase efficiency in the manufacture of EC devices, while maintaining their structural stability (i.e. their ability to withstand cracking and failure both during the manufacturing process and when installed in the field).
Traditional EC devices and the insulated glass units (IGUs) comprising them have the structure shown in FIG. 1A. As used herein, the term “insulated glass unit” means two or more layers of glass separated by a spacer along the edge and sealed to create a dead air space (or other gas, e.g. argon, nitrogen, krypton) between the layers. The IGU 18 comprises an interior glass panel 10 and an EC device 19. The EC device 19 is comprised of an EC stack 11 comprising a series of applied or deposited films on the EC substrate 12. The EC substrate 12 is traditionally comprised of glass which has been heat-strengthened or tempered.
To form the IGU 18, a glass panel, which will become the EC substrate 12, is first cut to a custom size according to the dimensions needed. The cut glass panel 12 is then tempered or heat-strengthened to provide sufficient strength to endure fabrication stresses and stresses encountered during its service life (“service loads”). The EC device stack 11, comprising, for example, a series of thin films, is then applied or deposited to the glass panel 12 by methods known in the art (see, for example, U.S. Pat. Nos. 7,372,610 and 7,593,154, the disclosures of which are incorporated by reference herein). Cutting of the glass panel 12 is not performed after tempering or heat strengthening. Likewise, the substrate of an EC device 19 is generally not tempered or heat-strengthened after the films forming the EC stack 11 are deposited (unless using a suitably post-temperable EC films stack and process). The IGU 18 is then assembled by combining the EC device 19 with another glass panel 10. The two panels are separated by spacers 17. Panel 10 may contain thin film coatings on either side (e.g. for solar control).